A third generation partnership project (3GPP) mobile system based on a wideband code division multiple access (WCDMA) radio access technology has been widely developed all over the world. A high-speed downlink packet access (HSDPA), which is a first step in the evolution of the WCDMA, provides the 3GPP with a radio access technology having high competitiveness. However, since radio access technology has been continuously developed in view of requirements and expectations of users and providers, evolution of a new technology in the 3GPP is required to increase competitiveness.
Accordingly, “Evolved UTRA and UTRAN” has been studied for the purpose of developing a wireless transmission technology that can significantly reduce cost while providing a high-quality service. The 3G long-term evolution (LTE) aims to reduce cost of a user and a provider and improve service quality as well as expanded coverage and system capacity improvement. The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure and an open interface, and adequate power consumption of a terminal as an upper-level requirement.
Generally, one Node-B is deployed in one cell. A plurality of user equipment (UE) may be located in one cell. The user equipment must perform a random access process for access to a network.
FIG. 1 is a block diagram illustrating a communication network, such as a network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed so as to provide a variety of communication services such as sound and packet data.
As illustrated in FIG. 1, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN) and a core network (CN). The E-UTRAN may include one or more evolved Node-B (eNode-B) 20. The CN may include a node for registering a user of a user equipment (UE) 10 and one or more E-UTRAN access gateway (AG) 30 positioned at the end of the network and connected to an external network.
As used herein, “downlink” refers to communication from an eNode-B 20 to the UE 10 and “uplink” refers to communication from the UE to an eNode-B. The UE 10 refers to communication equipment carried by a user and may be also be referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.
An eNode-B 20 provides end points of a user plane and a control plane to the UE 10. An AG 30 provides an end point of a session and mobility management function for the UE 10. An eNode-B 20 and an AG 30 may be connected via an S1 interface.
An eNode-B 20 is generally a fixed station that communicates with a UE 10 and may also be referred to as a base station (BS) or an access point. One eNode-B 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNode-Bs 20.
An AG 30 is also referred to as a mobility management entity/user plane entity (MME/UPE). An AG 30 may be classified into a portion for performing a user traffic process and a portion for performing a control traffic process. New communication may be performed between an AG 30 for performing the user traffic process and an AG for performing the control traffic process using a new interface.
An interface for distinguishing between the E-UTRAN and the CN may be used. A plurality of nodes may be connected between an eNode-B 20 and an AG 30 via the S1 interface. The eNode-Bs 20 may be connected to each other via an X2 interface and neighboring eNode-Bs 20 may always have a meshed network structure that has the X2 interface.
Layers of radio interface protocol between the UE 10 and the network may be classified into a first layer L1, a second layer L2 and a third layer L3 based on three lower-level layers of an open system interconnection (OSI) reference model that is widely known in communication networks. A physical layer belonging to the first layer provides an information transfer service using a physical channel. A radio resource control (RRC) layer belonging to the third layer serves to control radio resources between the UE 10 and the network. The UE and the network exchange an RRC message via the RRC layer.
The RRC layer may be located in a network node of an eNode-B 20 or AG 30. Alternatively, the RRC layer may be located at an eNode-B 20 or AG 30.
The radio interface protocol horizontally includes a physical layer, a data link layer and a network layer and vertically a user plane for transmitting data information and a control plane for transmitting a control signal. FIG. 2 is a block diagram illustrating the control plane of the radio interface protocol. FIG. 3 is a block diagram illustrating the user plane of the radio interface protocol. FIGS. 2 and 3 illustrate the structure of the radio interface protocol between the UE 10 and the E-UTRAN based on a radio access network standard.
As illustrated in FIGS. 2 and 3, the physical layer provides an information transfer service to an upper-level layer using a physical channel. The physical layer is connected to a medium access control (MAC) layer, which is an upper-level layer, via a transport channel.
Data is transferred between the MAC layer and the physical layer via the transport channel. Data is transferred between different physical layers, such as, a physical layer for a transmitter and a physical layer for a receiver, via a physical channel.
The MAC layer, which belongs to the second layer, provides a service to a radio link control (RLC) layer, which is an upper-level layer, via a logical channel. The RLC layer, which belongs to the second layer, supports reliable data transmission. It should be noted that the RLC layer is depicted in dotted lines, because if the RLC functions are implemented in and performed by the MAC layer, the RLC layer itself may not need to exist.
A packet data convergence protocol (PDCP) layer, which belongs to the second layer, performs a header compression function to reduce the size of an Internet Protocol (IP) packet header that includes unnecessary control information and has a relatively large size. In this way, efficient transmission of packets in a radio section having a narrow bandwidth may be facilitated when transmitting an IP packet, such as an IPv4 packet or an IPv6 packet.
The Radio Resource Control (RRC) layer, which belongs to the third layer, is defined in only the control plane. The RRC layer serves to control the logical channel, the transport channel and the physical channels in association with configuration, reconfiguration and release of radio bearers. A radio bearer is a service provided by the second layer for data transmission between the UE 10 and E-UTRAN.
A downlink transport channel for transmitting data from the network to the UE 10 includes a broadcast channel (BCH) for transmitting system information and a downlink shared channel (SCH) and shared control channel (SCCH) for transmitting the user traffic or a control message. The traffic or the control message of a downlink multicast service or broadcast service may be transmitted via the downlink SCH or a additional multicast channel (MCH).
An uplink transport channel for transmitting data from the UE 10 to the network includes a random access channel (RACH) for transmitting an initial control message and an uplink shared channel (SCH) and shared control channel (SCCH) for transmitting the user traffic or the control message. The RACH for transmitting the initial control message from the UE 10 to the network will now be described.
The random access process is performed via a random access channel (RACH) that is an uplink transport channel. The user equipment transmits an initial control message to the network via the RACH. The RACH is used to synchronize the user equipment with the network and to acquire radio resources when the user equipment needs to transmit data but has no uplink radio resources for transmitting the data.
More than one user equipment may try to acquire the same radio resources via the RACH. When this occurs, more than one user equipment may simultaneously transmit messages using the same radio resources. The messages may collide with each other and their transmission may fail.
A user equipment that fails to transmit a message uses the RACH again after a pre-determined time elapses. Data transmission time may significantly increase due to collisions and radio resources may be wasted due to re-access.